1. FIELD OF THE INVENTION
The present invention relates to a focus detecting apparatus having both a passive focus detection feature and an active focus detection feature, and being suited for use in a photographic camera, such as a single reflex camera, having an interchangeable lens capability.
2. DESCRIPTION OF THE PRIOR ART
A passive focus detecting apparatus suitable for use in a camera of the type referred to above is disclosed in, for example, the Japanese Laid-open Patent Publication No. 57-74423 first published in 1982. In this passive focus detecting apparatus, the focus detecting optical system is required to be so arranged and so constructed that the light flux received will not be vignetted by the exit pupil of the photo-taking lens. However, a variety of interchangeable lenses are available for a single reflex camera and the position and the size of the exit pupil varies from one interchangeable lens to another. Therefore, in order for the focus detection to be possible with any one of the available interchangeable lenses, the light flux must be received at a region adjacent the optical axis. However, this setting tends to bring about the following problem. Specifically, when the focus detection is carried out by causing the above mentioned focus detecting apparatus to receive light which has been projected onto an object through the photo-taking lens and subsequently reflected back from the subject, as when the active automatic focus (i.e., active AF) is carried out, it is difficult to avoid receiving harmful light which has been reflected between the surfaces of lens elements of the photo-taking lens and then returned toward the focus detecting apparatus without having been projected onto the object. On the other hand, since the number of usable lenses will be limited if the light flux to be received is so positioned and arranged as to be remote from the optical axis, the photo-taking region (the number of lenses usable) during the passive AF is limited to a value smaller than that hitherto available.
Japanese Laid-open Patent Publications No. 54-155832 and No. 57-22210 published in 1979 and 1982, respectively, disclose a focus detecting optical system wherein light emitted by a light projecting means is projected onto an object to be photographed through a portion of the photo-taking lens. The reflected light from the object is then received by a focus detecting means through a different region of the photo-takng lens. In the principal plane of the photo-taking lens, the optical axis for the projecting light and the optical axis for the receiving light do not assume a point-to-point symmetry with respect to the point of intersection between the principal plane of the phototaking lens and the optical axis of the photo-taking lens. In other words, the optical axis for the projecting light and the optical axis for the receiving light do not assume a point-to-point symmetry with respect to the optical axis of the photo-taking lens in the principal plane of the photo-taking lens, which avoids any possible influence which would be brought about by the harmful light. However, as hereinbefore described, a variety of interchangeable lenses having different minimum F-numbers and/or different focal lengths are available for a single reflex camera. In order for the focus detecting apparatus to be adaptable to any one of these interchangeable lenses, the receiving light flux must necessarily be received at a position adjacent the optical axis of the photo-taking lens. By way of example, where coverage is desired to be extended to the photo-taking lens having the minimum F-number of F. No.=5.6, a design must be made to receive the light flux of about F.No..gtoreq.7, even though the difference in position of the exit pupil among the individual photo-taking lenses, an error in adjustment of the focus detecting apparatus and the accuracy of fitting of the photo-taking lens to the camera body are taken into consideration. When the F-number of 7 is expressed in terms of the angle .theta. from the optical axis of the photo-taking lens, the angle .theta. will be about 4.1.degree.. Assuming that the light flux of an F-number within the range of 7 to 11 is used as the receiving light flux, the angle .theta. will be within the range of about 4.1.degree. to 2.6.degree., the smallest angle .theta. being 2.6.degree.. (See FIG. 10).
It is well known from, for example, the above mentioned Japanese Publication No. 57-22210 that a region of the harmful light radiated from a point on the optical axis of the photo-taking lens and reflected toward the focus detecting apparatus after having been reflected by a surface of the photo-taking lens assumes a point-to-point symmetry with respect to the optical axis of the photo-taking lens. Since the area in which the point-to-point symmetry takes place depends on the size of a source of light to be projected and the angle of the projecting light flux, it is necessary to reduce the size of the projecting light source and the angle of the projecting light flux if it is desired to reduce the area; that is, if it is desired to avoid having the harmful light fall on the focus detecting sensor.
The harmful light received on a light receiving surface is shown in FIG. 11. The size of the projecting light source is mainly related to the size x of the harmful light, and the angle of the reflected light flux is mainly related to the angle of spread of the harmful light. Summarizing the above, when the angle (to shown in FIG. 10 of the receiving light flux incident uponthe focus detecting apparatus becomes smaller, it becomes more necessary to reduce the size of the projecting light source and the angle of the projecting light flux. However, where the receiving light flux is received at a region adjacent the optical axis of the photo-taking lens as hereinbefore discussed, the error in adjustment of the focus detecting apparatus, the error in accuracy of fitting the photo-taking lens to the camera body and the error in eccentricity of the photo-taking lens itself make it difficult to avoid the harmful light. Also, the excessive reduction in size of the projecting light source and the angle of the projecting light flux would result in an excessive reduction in the amount of light to be projected, rendering the distance over which the distance measurement is possible to be small.
The Japanese Laid-open Patent Publications No. 59-40610, No. 59-65814, No. 59-129810, No. 59-129812 and No. 59-195607, all published in 1984, disclose an arrangement wherein at least two sets of light deflecting members are disposed in a secondary image forming system so that rays of light from outside of the exit pupil of the photo-taking lens and rays of light from inside thereof can form respective images at different positions on a substrate for a light receiving element. An arrangement has also been made whereby a set of light receiving elements for receiving the rays of light from the outside of the exit pupil and a set of light receiving elements for receiving the rays of light from the inside of the exit pupil can be switched over selectively depending on the aperture value of the photo-taking lens. According to this system, the light deflecting members are used as a means for differentiating the rays of light from the outside of the exit pupil and those from the inside of the exit pupil. Since they merely divide the exit pupil into an inside region and an outside region, no consideration has been given to the harmful light which would occur during the previously discussed active AF. By way of example, the light dividing means, the image of which has been formed in the vicinity of the exit pupil of the photo-taking lens by a field lens, is shown in FIG. 12. Referring to FIG. 12, reference characters 1a, 1b and 2a, 2b represent images of two light deflecting portions of the light dividing means projected in the vicinity of the exit pupil of the photo-taking lens, which images represent a generally square shape since the overall shape of the light deflecting member is square. With this shape, it is clear that the harmful light cannot be effectively eliminated even with the rays of light passing through the outside of the exit pupil. Moreover, since the light deflecting members are used, there may arise harmful light which would be reflected by an edge portion of the light deflecting member. For this reason, there is a problem in that the outside portion and the inside portion cannot be brought close to each other (the outside portion must be set outside more than necessitated).
The Japanese Laid-open Patent Publication No. 60-98413, published in 1985, discloses an arrangement wherein the pupil of the photo-taking lens is divided into first to third pupil areas by a dividing means, the first pupil area being used to pass a projecting light flux therethrough so as to be projected onto the object to be photographed through a central area of a focus detection field opening, and wherein an image of the object formed by the photo-taking lens within the focus detection field opening is formed on a plurality of separate sensors through the second and third pupil areas. According to this publication, the optical axis of the light projecting system is positioned within a plane delimited by the longitudinal direction of the focus detection field opening and the optical axis of the photo-taking lens. Both the projecting light region and the receiving light region are positioned outside the area of point-to-point symmetry with respect to the optical axis of the photo-taking lens. In other words, as shown in FIG. 13, the first pupil area which is the projecting light region and the second and third pupil areas which are the receiving light region are set so as to divert from the substantially symmetrical relationship with respect to a vertical line component perpendicular to the optical axis of the photo-taking lens. Accordingly, it is not suited for use in carrying out AF with the photo-taking lens having a relatively large minimum F-number. In other words, reduction in size of the light receiving region results in reduction of the amount of light received, which is undesirable, particularly for the active AF.